Historically, epoxy resins are known for excellent electrical insulation and adhesion properties. Bisphenol-A based epoxy resins are the most common among various types of epoxy resins available commercially. Curing of epoxy resin is achieved by reacting it with polyamines, polyamides, carboxylic anhydrides, poly-phenols, and amine complexes. The epoxy or the ‘oxirane’ ring opens up and reacts with the hardener without forming any byproduct at ambient or elevated temperatures depending upon the nature and use of hardeners.
Recently, epoxy foams have been investigated because they exhibit excellent combination of mechanical properties like tensile strength, adhesion strength, compression strength, electrical properties like dissipation factor, dielectric constant, volume resistivity and adhesion properties including low shrinkage, low density, low water absorption and low flammability.
In order to make epoxy foams, suitable foaming agents are required. The foaming agents may be chemicals such as carbonates, bicarbonates, hydrazides and the like which decompose during exothermic reactions and liberate gases. The chemical agents may also react with resins/hardeners and release the gases necessary for the foaming action. Foaming agents can also be physical agents such as toluene, fluorocarbons and the like which simply vaporize during exothermic reactions and the liberated vapors help in forming foam structures. Chemical surfactants also play important roles in mixing or dispersing of foaming agents, foam cell sizes and structures. Surfactants may be silicon based or polyol based materials. Micronized inert fillers of particle size in nanometers are also very important to control cell size, cell structure, and foam density by nucleating effects. Higher concentration of fillers also helps in reducing cost and exothermic temperatures.
Processing of epoxy foams is a challenging task and needs synchronization of several parameters such as rheology of the mix, curing rate, and rate of reaction of foaming agents. An appropriate rise in the viscosity during reactions allows formation of uniform cell structures. Too low and/or fast rise in the viscosity restrict the development of foam structures and that may collapse completely. Too fast and/or too slow reactions between resins and curing agents also spoil the network and cause the foams to be irregular, in appropriate in density, and with disturbed cell structures.
The density of the foam is dependent on several factors such as the selection of resins, hardeners, types and amount of foaming agents, fillers, and processing parameters like temperature, mass of the mix, and mold shapes and sizes. Epoxy foams with density up to 170 kgs/m3 are reported with silicon type blowing agents.